A newly discovered mutator gene or controlling element with a pronounced effect on viability in D. melanogaster provides a valuable opportunity for the study of mutation. One broad objective of this proposal is to make a detailed investigation of the genetic properties of the mutator gene and its induced effects. A second major objective is to exploit the potential offered by the mutator gene in experiments that require a high rate of spontaneous mutation. In particular it provides an exceptionally good opportunity to study several important aspects of fitness.